1. Field of the Invention
The present invention relates to a mold for injection molding a preform having an undercut lip portion and a plastic vessel, which is biaxially stretch blow molded from such a preform.
2. Description of the Related Art
A roll-on type vessel is known which receives a ball therein at the open top. The fluid content of the vessel can be applied to a sheet of paper by holding the vessel upside down and moving the ball received in the open top of the vessel on the sheet of paper in rolling contact therewith. When such vessel is to be molded of plastic material, the open top of the vessel must be of an undercut configuration which extends inwardly, proceeding in a direction toward the open top of the vessel.
In each case when a hollow vessel is to be injection blow molded or stretch blow molded, a preform having a cylindrical closed-end barrel portion and a lip portion formed therein at the open top of the barrel portion is first injection molded. Pressurized air is then introduced into the interior of the preform barrel portion to blow mold a plastic vessel. In the stretch blow molding process, a stretching rod also is used to stretch the preform in its longitudinal axis while being blow molded.
In the stretch blow molding process, the injection molding of the preform is carried out by the use of an injection core mold for defining the lip and inner barrel wall of the preform, an injection cavity mold for defining the outer barrel wall of the preform and a lip cavity mold for defining the outer wall of the lip portion. Molten resin material is injected into a space defined between the inner cavity walls of the injection and lip cavity molds and the outer wall of the injection core mold to form a preform. Thereafter, the molded preform will be removed out of the injection cavity mold, for example, by moving the injection core mold and the lip cavity mold in the upward direction, with the injection core mold being drawn out from the interior of the molded preform.
In order to form a roll-on type vessel, the lip portion of the preform must have an undercut which extends inwardly proceeding in a direction toward the open top of the vessel. Thus, an undercut area will be defined between the lip cavity mold and the core mold.
However, such a mold structure disables the release of a molding from the mold. Namely, the undercut of the molded preform as well as the mold will be damaged when the injection core mold is drawn out from the interior of the molded preform. At this time, the preform is held only by a pair of lip cavity mold sections which can be moved to their closed or opened position. Therefore, these lip cavity mold sections may not, be moved away from each other during the injection molding process. A space where the preform can resiliently deform at its undercut will not be around the preform at all since the preform is held by the lip cavity mold on releasing the injection core mold.
In the injection blow molding process, the injection core mold also functioning as means for introducing the blow air is lastly separated from the blow molded vessel. Thus, it can secure a space where the undercut area of the lip portion can be resiliently deformed. However, the plastic materials which can be released by resiliently deforming such an undercut area in the prior art injection blow molding process are limited to relatively soft plastic materials such as polypropylene, polyethylene and the like. The mechanical strength and other properties of vessels formed by the injection blow molding process are inferior to those of vessels formed by the stretch blow molding process since the vessels are not biaxially stretched in the injection blow molding process.